1. Field of the Invention
The present invention generally relates to a method for manufacturing a metal-induced-laterally-crystallization thin-film transistor, and more particularly to a method for forming a thin film transistor with lateral crystallization.
2. Description of the Prior Art
A method of crystallizing amorphous silicon using heat treatment at a low temperature after a certain kind of a metal layer has been deposited on the amorphous silicon is know as an MIC process. The MIC process is beneficial due to the low temperature crystallization of amorphous silicon. However, the MIC process has not been applied to electronic devices because of an inflow of metal into the thin film of crystallized silicon formed underneath the metal layer, which causes the intrinsic characteristics of amorphous silicon to deteriorate.
FIGS. 1A to 1C show a method of fabricating a channel region of a thin film transistor using an MILC process according to a related art.
Referring to FIG. 1A, an amorphous silicon layer 110, as an active layer is deposited on an insulation substrate 100 having a buffer film (not shown in the above figure) on its upper part, and the active layer 110 is patterned by photolithography and an etching process. A gate insulation layer 120 and a gate electrode 130 are formed on the active layer by conventional processes.
Referring to FIG. 1B, a nickel layer 140 is formed to a thickness of 10 angstrom by sputtering nickel on the entire surface of the formed structure. Then a source region 110S and a drain 110D are formed at portions of the active layer by heavily doping the entire surface of the formed structure with impurities. Between the source region 110S and drain region 110D, a channel region 110C is formed on the substrate 100.
Referring to FIG. 1C, amorphous silicon in the active layer is crystallized by heating the substrate 100 at a temperature of 350xc2x0 C.-500xc2x0 C. Then the source region 110S and drain region 110D on which the nickel layer 140 has been formed become the MIC regions having silicon crystallized by an MIC process. The channel region 110C without the nickel layer 140 formed directly thereon becomes the MILC region where silicon has been crystallized by an MILC process. Impurities are activated in the source region 110S and drain regions 110D during the heat treatment as amorphous silicon is crystallized in the active layer.
In the thin film transistor fabricated by the above-described method according to the conventional art, the channel region 110C has boundaries defined by the crystalline structure of silicon in the MIC regions facing that of silicon in the adjacent MILC region. Since the boundary between the MIC region and the MILC region is located at the junction where the source or drain region meets the channel region, an abrupt difference in the crystal structure appears in the junction and the metal from the MIC region contaminates the adjacent MILC region. Consequently, a trap is formed at such junctions as soon as the TFT is turned on which causes unstable channel regions and deteriorates the characteristics of the thin film transistor.
Accordingly, it is desired to provide a method for forming thin film transistor with lateral crystallization, to not only improve planarity of interface, increase grain of big polycrystalline silicon channel and lateral grain growth and carrier mobility, but also to overcome the drawbacks of the conventional grain formation process.
In accordance with the present invention, a method is provided for forming thin film transistor with lateral crystallization that substantially can be used to improve the carrier mobility issue in the conventional process.
One of the objectives of the present invention is to provide a method to form a thin film transistor with lateral crystallization to improve the carrier mobility issue.
Another of the objectives of the present invention is to provide a method to form a thin film transistor with lateral crystallization using a very simple process, having a low temperature and maintaining high carrier mobility.
A further objective of the present invention is to provide a method to form a thin film transistor with lateral crystallization improving the planarity of the interface, increasing the grain of the big polycrystalline silicon channel, and also increasing lateral-growth grain.
In order to achieve the above objectives, the present invention provides a method for forming a thin film transistor with lateral crystallization. The method at least includes the following steps. First of all, an insulation substrate is provided. Then, an amorphous silicon layer is provided on the insulation substrate. The seeds are formed by annealing a portion of the amorphous silicon layer by excimer laser system, and the lateral-growth grain is formed by using the seeds to grow laterally by annealing the amorphous silicon layer, wherein the amorphous silicon layer defines an active region. Then, sequentially a dielectric layer and a polysilicon layer are deposited on the active region, wherein the dielectric layer and the polysilicon layer are gate electrodes, a gate is defined on the substrate, and the polysilicon layer is formed by etching. Next, source and drain regions are formed by implanting numerous ions into amorphous silicon layer by using the gate electrode as a mask.